Remove all or nearly all the scales from a wing and note the arrangement of the veins (venation). Compare with venation in wings of locust.

Make drawing showing venation in the butterfly's wings.

Fig. 42.—Bit of wing of Monarch butterfly,
Anosia plexippus, magnified to show the
scales; some scales removed to show the
insertion-pits and their regular arrangement.
(From specimen.)

The venation of insects' wings is much used in insect classification, and the various veins have been given names. The names of the veins in the butterfly's wings are given in fig. [43]. When the veins in the wings of all the various groups of insects are studied, it is evident that the principal ones are the same in all insects, so that the costa, sub-costa, radius, media, cubitus and anal veins of the butterfly's wings can be compared with the corresponding veins in the wings of a beetle or wasp or fly. Noting the differences in the number and character of branching of these principal veins, and the number and disposition of the cross-veins which connect the longitudinal veins, the various kinds of insects can be to a large extent properly grouped or classified. A detailed account of the wing-veins of insects is given in Comstock and Kellogg's "Elements of Insect Anatomy," chap. VII.

Fig. 43.—Wings of monarch butterfly,
Anosia plexippus, to show venation; c,
costal vein; sc, sub-costal vein; r, radial
vein; cu, cubital vein; a, anal veins.
In addition most insects have a vein
lying between the sub-costal and radial
veins called the median vein.

Of how many segments is the abdomen composed? The first or basal segment is depressed, while the others are more or less compressed. The spiracles are, as in the locust, situated on the lateral aspects of the abdominal segments. What segments bear spiracles? The terminal segments of the abdomen differ in the two species. In the female the dorsal part of the (apparently) last segment is longer than the ventral part and is bent down over it forming a sort of hood over a space enclosed partly by this hood, partly by a bluntly-pointed projection from the ventral surface, and partly by the lateral margins of the segment. In this chamber lies the opening from which the eggs issue. In the male there are several backward-projecting, horny, thin processes.